The proposed research will explore the importance of particle size fractions and temporal aspects of exposure to coal mine dust to the development of chronic bronchitis and airways obstruction in underground coal miners. This research offers several strengths compared to previous studies: both cumulative respirable and tracheobronchial dust exposures will be addressed, personal air sampling data will be utilized and smoking history will be quantitative (pack-years). Thus, this study should be able to significantly improve scientific understanding of several critical aspects of the relationship of dust exposures among coal miners to the development of respiratory disease. The methodology employed will be as follows. Particle size distributions for coal dust exposures will be obtained using personal cascade impactors. Distributions will be characterized for major occupational titles for miners working at the coal face in continuous, longwall and conventional mining sections and for non-face mining occupations. The distributions will be used to define respirable and tracheobronchial dust fractions for each occupation. The fractions will then be used in conjunction with work histories and a matrix of respirable dust concentration currently under development to estimate several indices of cumulative exposure. The indices will include respirable and tracheobronchial exposures weighted with uniform temporal weights, with higher weights to earlier exposures and with higher weights to recent exposures. Finally, exposure-response analyses will be conducted using a previously identified subcohort of NIOSH's National Study of Coal Workers' Pneumoconiosis to explore the relative importance of the fine and coarse fractions of dust and the temporal aspects of exposure on the development of chronic bronchitis and airways obstruction.